In a conventional control apparatus for controlling a shift operation in an automatic transmission, torque transmitted to an off-going friction engagement element, which is adapted to be disengaged at a time of shifting in the automatic transmission, is required to be reduced in accordance with increase of torque to be transmitted to an on-coming friction engagement element, which is adapted to be engaged at the time of shifting. A one-way clutch has been employed for reducing the torque transmitted to the off-going friction engagement element. However, somewhat recent developments have led to a control apparatus for controlling a shift operation in an automatic transmission, in which a clutch-to-clutch shift operation is performed by controlling oil pressure to be supplied to a clutch for the on-coming friction engagement (hereinafter, referred to as an engaging clutch) and a clutch for the off-going friction engagement element (hereinafter, referred to as a disengaging clutch) by use of a software. In this case, a shift stage is switched in the automatic transmission by controlling each engaging and disengaging clutch in mutual relation thereto.
According to the clutch-to-clutch shift operation by the control apparatus for controlling the shift operation in the automatic transmission, a shift feeling may deteriorate unless switching operation between the disengaging clutch and the engaging clutch is performed with high precision. Described below are examples of the unfavorable switching operation therebetween. When the torque to be transmitted to the engaging clutch is excessively increased relative to the decrease of the torque transmitted to the disengaging clutch, the automatic transmission may lapse into an interlocked condition, wherein torque for rotating an output shaft of the transmission is decreased. On the other hand, when the torque to be transmitted to the engaging has not been sufficiently increased relative to the decrease of the torque transmitted to the disengaging clutch, torque at a side of a turbine runner of a torque converter can not be appropriately transmitted to the automatic transmission such that racing of the turbine runner may occur. In this case, the shift feeling may deteriorate.
In order to overcome the above descriptions, in a conventional control apparatus for hydraulically operated vehicular transmission disclosed in a U.S. patent published as U.S. Pat. No. 6,007,458 corresponding to a Japanese Patent Laid-Open Publication No. 10-153257, the disengaging clutch has been designed to slip within a range of a predetermined gear ratio of the turbine rotational speed and the output shaft rotational speed,. Therefore, the disengaging clutch sensibly slips depending on a change of a transmission condition due to the slip. The control apparatus judges that the transmission condition has transferred from a torque phase to an inertia phase when a slip amount in the transmission is decreased. At this point, the oil pressure to be supplied to the engaging clutch is increased, and at the same time the oil pressure supplied to the disengaging clutch is immediately drained. Therefore, an engine (i.e. the turbine runner) can be effectively prevented from racing and the transmission condition can be transferred to the inertia phase at an early stage.
In a conventional gear shift control device of an automatic transmission disclosed in a Japanese Patent Laid-Open Publication No. 11-182663, the disengaging clutch slips by setting a speed for decreasing the oil pressure supplied to the disengaging clutch faster than a speed for increasing the oil pressure to be supplied to the engaging clutch. When the slip amount of the disengaging clutch reaches a predetermined amount, the transmission condition is transferred form the torque phase to the inertia phase by rapidly decreasing the oil pressure supplied to the disengaging clutch. Therefore, the transmission can be prevented from an inter lock condition.
Further, according to a conventional control apparatus for controlling a shift operation in an automatic transmission, which has been suggested by the applicant of the present invention, the disengaging clutch slips and a sip slip amount is maintained at an appropriate slip amount until the engaging clutch can be transmitted with sufficient torque. A racing amount of a turbine rotation is referred to for maintaining the slip amount at the appropriate slip amount. The racing amount of the turbine rotation can be determined by subtracting a value, which is calculated by multiplying an output rotational speed No by a gear ratio Geari of an actually selected shift stage, from a turbine rotational speed Nt. The feedback control can be performed for controlling the racing amount of the turbine rotation (=Nt−No.Gear1) to follow a target value.
In this case, the disengaging clutch has been designed to be released from the engaged condition and the engaging clutch is transferred to the disengaged condition along with control of the racing amount of the turbine rotation. Therefore, the transmission can be effectively prevented from the inter lock condition and the shifting feeling can be enhanced. Even when the increase of the oil pressure to be supplied to the engaging clutch is delayed, the oil pressure supplied to the disengaging clutch can be controlled at a preferable oil pressure level until the engaging clutch becomes ready for being supplied with the sufficient oil pressure. Therefore, the control apparatus can effectively prevent the rotation of the engine or the turbine runner from largely racing such that the shift feeling can be effectively prevented from becoming worse.
The output torque as one of the reference for judging the shifting feeling fluctucates depending on change of the turbine rotational speed, i.e. depending on acceleration of the turbine rotation. Accordingly, the output torque may fluctuate unless the slip amount of the disengaging clutch is appropriately controlled, so that the shift feeling may deteriorate.
According to the conventional control apparatus disclosed in the U.S. patent published as U.S. Pat. No. 6,007,458, the slip amount lies within the range of the predetermined speed ratio such that the slip amount just drifts as it is during a transient state until the gear ratio reaches the predetermined gear ratio. Thereore, the slip amount and the output torque can not be substantially controlled until the gear ratio reaches the predetermined gear ratio.
According to the conventional gear shift control device disclosed in the Japanese Patent Laid-Open Publication No. 11-182663, an actually detected slip amount referred to for performing the feedback control is identical to a target slip amount (i.e. a target value). Therefore, the slip amount can not be controlled immediately after the commencement of the feed-back control and the slip amount is modified to the target value after occurrence of overshooting. Therefore, it may cause backlash of an output shaft of the transmission.
A need thus exists for providing an improved control apparatus for controlling a shift operation in an automatic transmission and a method of designing the same.